Blind fasteners are used in a variety of applications to connect two or more workpieces together. In the construction of aerodynamic designs, such as control surfaces on aircraft and the like, a substantially flush surface usually is desired on the accessible side of the panels. Often, however, access to the blind side is not possible, which can complicate fastener installation. In these cases, the use of a blind fastener is appropriate, since access to only one side of the panel is available to install the fastener.
Typical blind fasteners comprise an internally threaded nut body and an externally threaded cylindrical core bolt or stem passing in threaded engagement through the nut body. The inserted end of the core bolt has an enlarged core bolt head while the other end of the core bolt has a wrench-engaging portion. Thus, upon insertion of the fastener into the aligned apertures of a pair of workpieces and upon turning motion of the core bolt relative to the nut body, the core bolt is moved in an axially outward direction through the nut body. This axially outward movement typically causes a deformable sleeve around the core bolt and intermediate the nut body to deform around the tapered nose of the nut body. The deformable sleeve forms a bulb-like shape to provide a blind side head against the inner surface of the inner work piece. The core bolt further is provided with a localized weakened region or break groove adapted to sever the core bolt at a predetermined torque and location.
It is advantageous that the break groove shears the core bolt in a substantially flush relation to the fastener body head after the fastener is fully set. Particularly, accurate core bolt break is sought for fasteners having countersunk body heads to provide a flush relationship between the set fastener and the outer panel, thus providing a smooth aerodynamic surface after the fastener is set.
However, due to numerous factors including variations in combined panel thickness, sometimes the break groove on the core bolt extends beyond a flush position with the fastener body head. Therefore, when shear or breakage occurs at the break groove, a portion of the remaining core bolt protrudes beyond the fastener body head. As a result, it is often necessary to grind the protruding core bolt so that the core bolt is flush with the fastener body head. Prevention of such protrusion will provide a cost savings through the elimination of additional operations and manpower required in shaving, smoothing, and trimming the protruding core bolt stem to provide a flush finish.
Conversely, positioning the break groove to break flush to below the head surface can result in cavities that must be filled. Again, eliminating the need to fill such cavities will provide a cost savings through the elimination of additional operations and manpower required to provide a flush finish. In addition, low breaks (below flush) may result in some loss of strength in the fastener head.
Additional requirements for fasteners are required when used on aircraft. For example, components of multi-piece mechanical fasteners used on aircraft forward of the engine air inlets must be mechanically locked to prevent unthreading. As a result of mechanically locking the fasteners, the likelihood of such fasteners becoming loose and being subsequently ingested by the engine is reduced. Conventional non-blind threaded fasteners are typically lock-wired to achieve this goal. Lower-strength blind fasteners, on the other hand, often incorporate a locking ring to achieve this goal. Both of these approaches, however, require additional steps in the process of installing the fasteners into the aircraft and/or require additional components to be included with the fasteners. Further, it is not economically feasible to incorporate a locking ring into a high strength blind fastener; particularly the threaded type described herein.
Generally, structural joints should have strength at least equivalent to the panels in which they are installed. Otherwise, the fasteners will fail before the panels fail. As airframe joints are designed to carry shear loads, the joint shear strengths should correspond with the structure material bearing load strength. The shear load capability of a structural joint is usually measured using Metallic Materials Property Development and Standardization (FAA/DOD MMPDS) guidelines and testing in accordance with MIL-STD-1312 Test Method #4. A load versus elongation plot of a single fastener joint is shown in FIG. 5. Generally, the higher the yield strength and ultimate strength (i.e., higher curve), the more suitable the fastener is for structural applications.
Having a relatively large residual clamp load in the joint enhances structural strength. In addition, the large residual clamp load allows fasteners to close gaps between panels and keep the panels tightly clamped together as desired. High residual clamp loads also reduce microscopic movement between metal panels during flight operations, thereby minimizing the likelihood that fretting and fatigue cracks will develop.
Laminated carbon fiber composites are becoming increasingly prevalent in airframe structure. Primarily, laminated carbon fiber composites provide lighter weight and accompanying fuel savings. Composites, however, cannot endure the high compressive stresses induced by the installation of conventional fasteners designed for metallic structure. It is, therefore, desired to spread the fastener clamp loads over a large region on the panels to minimize contact stresses while maintaining high clamp loads.
Existing blind fasteners incorporating a locking ring fail to provide one or more of the above-mentioned features. Pull-type blind bolts, for example, incorporate band-annealed sleeve components for a controlled upset against the panel surface. This softening of the sleeve necessarily extends into the shear plane, reducing joint strength and/or stiffness. In addition, the pull-type nature of these fasteners causes a majority of the installation clamp load to relax upon the fracture of the pintail during installation. Specifically, the pintail of the pull-type fasteners tends to recoil upon fracture of the pintail.
Existing blind rivets induce minute clamp load or sheet gathering capability and have an extremely small blind side upset compared to blind bolts. These are generally suitable for low-strength applications in secondary metal structure (e.g., control surfaces), but are not strong enough for highly loaded primary structure (e.g., fuselage and wing joints).
Additional information will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The present invention overcomes these deficiencies of known fasteners. One of ordinary skill in the art will also appreciate additional advantages of the present invention.